Note: Descriptions are shown in the official language in which they were submitted.
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ECONOMICAL AND T}IERMALLY EFFICIENT CRYOPUMP
PA~EL AND PANEL ARRAY
TECHNIC'AL FIELD
The present invention pertains to capturing gas
molecules on extremely cold surfaces from enclosed volumes of
low pressure to create ultra-high vacuums, In particular the
invention relates to a unique cryopanel and cryopanel array
adapted to maintain high pumping speeds for hydrogen while
simultaneously pumping large quantities of argon and air.
ACKGROUND OF THE PRIOR A~T
The prior art of cryopumping (cryogenic pumping) is
adequately set out in the specification of U.S. Patent
4,150,549, and reference may be had to that patent for such
information. The '549 patent discloses one type of panel which
is ideally suited for the coldest end of an elongated
refrigerator to pump, among other things, hydrogen, argon and
air. U.S. Patent 4,219,5~8 discloses a method for improving
the cryopumping apparatus of the '549 patent while U.S. Patent
4,277,951 discloses a low profile cryopumping apparatus. U.S.
Patent 4,121,430 is representative of a number of cryopumps
with panels of varying configuration on the cold end of the
cryogenic refrigerator.
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U.S. Patent 4,295,338 discloses and claims a
cryopanel array oE the type which is cumbersome and difficult
to fabricate and not overly thermally efficient, of which the
present invention is a vast improvement.
BRIEF SUMMARY OF THE INVE~TION
The present invention relates to a cryopump and in
particular to a cryopanel designed for the second or coldest
stage of a two-stage cryogenic refrigerator of the dlsplacer
expander type wherein the panel geometry is a vertically tiered
conical array with linearly increasing base diameters from the
cold end of the refrigera-tor toward the warm stage of the
refrigerator with tapered bayonet joint interlocking cryopanel
sections to permit ease of assembly while mainta.ining good
thermal contact between the sections. An individual panel
geometry according to the present invention is able to maintain
extremely high hydrogen pumping speeds while simultaneously
pumping large quantities of argon and air. A cryopanel array
according to the present invention features ease of assembly,
lower cost, and thermal efficiency heretofore unknown with
prior art devices of ap~arently similar construction.
According to one embodim~nt of the present invention,
there is thus provided, a cryopump of the type having an
elongated refrigeration source adapted to mount and cool a
cryopanel, the improvement comprisiny: a plurality of
cryopanels of each being a surface of revolution ha~ing a
tapered bayonet interface portion being of generally
cylindrical shapte, the walls of the cylinder tapering slightly
from a first end of the interface portion to a second end of
the interface portion and a major pumping surface portion being
a continuation of the interface portion and being in the shape
of a truncated cone with a relatively flat angle between the
base and the wall of the cone; the cryopanels fabricated with
differing diameters for the base of the pumping surface whereby
the plurality of cryopanels are fixed by suitable means to the
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refrigeration source in a vertical array with the smaller
diame-ter panel first and the larger diameter panel last
installed beginning at the coolest end of the refrigeration
source.
In a still further development, there is also
provided a cryopanel being a surface of revolution having a
tapered bayonet interface portion being of generally
cylindrical shape, the walls o~ the cylinder tapering slightly
from a first end of the interface portion to a second of the
interface portion and a maior pumping surface portion being a
continuation of the interface portion and being in the shape of
a truncated cone with a relatively flat angle between the base
and the wall of the cone.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a fragmentary front elevational view
partially in section of a device according to -the present
invention.
Figure 2 is an enlarged and exaggerated diagram of
the interlocking mechanism for the cryopanel array of figure l.
Figure 3 is a fragmentary front elevational view of a
prior art apparatus illustrating agon cryodeposition on the
cryopanel surfaces.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
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In the Semiconductor Industry during recent years
cryopumps have become accepted as means for creating a
vacuum in much of the process equipment currently used
in the fabrication of large-scale integrated circuits.
Acceptance of the cryopump is due largely to its high
pumping speed and the elimination of thè potential for
oil contamination which is prevalent with diffusion
pumps heretofore used to create the vacuum environment.
It has been a concern of the industry that the present
crypumps require far too much regeneration because the
cryopump is basically a "capture" pump wherein accumula-
tion of cryo-deposited and adsorbed gases ultimately
leads to a need to rid the pump of the gases thus
captured. Prior art pumps which require frequent
regeneration have a severe limitation since the produc-
tion rate of integrated circuit chips can not easily be
maintained while the cryopumps are being regenerated.
Current commercially available crypumps which
apparently have high capacities for most of the commonly
encountered process gases share the common problem that
the time interval between regeneration often decreases
when two or more gases are being pumped simultaneously.
This is frequently encountered in sputtering equipment
where both argon and hydrogen are pumped simultaneously
and regeneration is prompted by a noticeable drop-off
in the hydrogen pumping speed. This decrease in speed
can result from either contamination of the hydrogen
adsorbent by abundant argon molecules or the plugging
of the hydrogen passages by the cryo-deposited argon.
Plugging creates a drop-off of hydrogen molecular
conductance and subsequent decrease in speed.
Referring to Figure 1, there is shown a cryogenic
refrigerator 10 of -the displacer expander type suitable
for use in cryopumping applications. Such a refrigerator
is disclosed and claimed in U.S. Patent 3,620,029 and
is sold under various model designations by Air Products
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and Chemicals, Inc. under the Trademark DISPLEX. The
refrigerator operates on a modified Solvay cycle producing
refrigeration in the order of 77K (Kelvin) at the base
of heat station 12 of the first stage 14 and refrigeration
of 10-20K at heat station 16 of second stage 18. The
cryopanel array of the present invention is built from
independent conical surfaces of revolution which are
interlocked, bayonet fashion, one with ~he other. Each
surface of revolution, for example, cryopanel 20 of the
array of figure 1, includes a first portion 22 in the
form of a tapered cylindrical adaptor or bayonet section
which begins on the first end 26 and tapers outwardly
toward a second end 24 which as a continuous surface 28
in the form of a cone with a relatively flat angle
between wall 2~ and base 29. The next cryopanel 30 is
identical to cryopanel 20, but is of smaller outside
diameter for the conical portion. The next panel in
the array 40 has the same overall configuration, but is
also of smaller diameter at the base of the cone than
panel 30. The uppermost panel 50 is again of smaller
diameter at the base of the cone than panel 40 and also
includes a closed top 48 which can be placed on heat
station 16 so that good thermal contact can be maintained
between heat station 16 and cryopanel 50 and in turn
the increasingly larger diameter cryopanels can be
supported by panel 50. While panel 50 is fabricated
with a closed top it could be identical to the other
panels (e.g., 20, 30, 40) and its adaptor or bayonet
section 52 disposed around the circumference of heat
station 16. Referring to figure 2, the method of
putting the panels together is based upon the overlapped
bayonet joint which is shown greatly exaggerated in
figure 2. For a given panel thickness, t, and tapered
bayonet angle, oC , the overlap, 1, of the adjacent
panel is given by the formula:
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1 = t/tan~
The spacing between panels, h, is given by the
formula:
~ h = t/sin d~
The contact surface area Ac in the bayonet contact
region between panels is determined by the formula:
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Ac = ~ (2r ~ 2t cos o{ + 1 sinc~ )V~ h2 -t 12 sin 2
For a cryopanel wherein the radius r is to be
0.975 in., ~ , equals 2.2 and the geometric parameters
are:
1 eguals 0.65 inches (16.5 mm) h = 0.65 inches
(16.5 mm) Ac equals 4.14 sq. in. (26.7 cm2). The
modular construction technique permits application of
charcoal or other adsorbent to the interior conical
surface (e.g. interior wall 28 of cryopanel 20) of each
cryopanel in the array prior to assembly. This technique
also makes it possible to include a layer of charcoal
in the outer exposed portions of the first or tapered
cylindrical portion 22 of panel 20 and the other tapered
portions (34, 44, 54 of panels 30, 40, 50) which are
exposed for the succeeding tapered cylindrical portions
(32, 42, 52 of panels 30, 40, 50) of the cryopanel
array. The modular construction permits ease of appli-
cation of charcoal and positive interlocking of sections
without the need for an excessive number of fasteners
or solder. In point of fact, it would be possible to
spot weld the sections together with a minimum number
of welds, thus enhancing rather than decreasing heat
transfer capability of the cryopanels. Utilization of
a tapered bayonet interface with a small taper angle, ~ ,
and generous overlap dimension, 1, generates high
interface contact stresses and large contact surface
areas, both of -these boundary conditions reducing the
effects of thermal contact resistance which must be
minimized in order to reduce the temperature difference
between any two points on the cryopanel array. Further
reduction in cryopanel temperature difference may be
achieved by putting a thin coating of a high thermal
conductivity medium such as high thermal conductivity
epoxy on the contact area between panels. A large
degree of adjacent bayonet joint overlap is used to
insure the continuity and wall thickness of the composite
tubular core which is necessary to convey heat from the
outer reaches of each panel to the heat sink provided
by the refrigerator heat station.
Positive loc~ing of adjacent sec-tions is most
economically insured by spot welding at two or three
points within the overlapping position of the tapered
bayonet joint at a position indicated as line "a" of
figure 1. While spo-t welding is preferred, riveting,
punch pricking, screwing or bol-ting along -the same
joint can also be used. Alternatively, the entire
assembly may be locked together by the use of slender
axial bolts which are passed through the inside of the
entire structure and used to hold the entire array in a
state of compression. The use of tapered bayonet
joints while illustrated as a means of assembling the
cryopanel for a commercial cryopump, can be used to
assemble any cryopanel array which is axially symmetric
geometry. The joint and panel geometry described lends
itself to economical mass production techniques such as
metal spinning and hydroforming.
The cryopanel array illustrated in figure 1 consists
of a vertical tier of conical sections with the linearly
increasing base diameters from the heat station 16
toward the heat station 12 of the refrigerator 10. The
conical silhouette of this array provides a large
frontal surface area when viewed from the cryopump
inlet louver 80 while allowing adequate protection for
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the charcoal from premature argon contamination. The
device of figure 1 includes a top or cover panel 70
which is fastened to -the heat station 16 by suitable
fasteners such as bolts 72 and 7~. Sandwiched between
top panel 70 and top of cryopanel 70 and also between
the bot-tom of croypanel 70 and top flat surface of the
2nd stage heat station 16 are indium gaskets 48 which
are used to enhance the transmission of heat. The
number of panels in a given array depends upon the
application so that there is enough charcoal surface
area to ensure high hydrogen capacity while providing a
large enough gap between adjacent surfaces to prevent
plugging of the gaps. According to tests, five conical
sections provide a good balance between -these requirements.
As is well known in the art, a louver 80 by means
of a housing or second-stage panel 82 is thermally
connected to the warmer or second stage heat station 12
of the refrigerator 10. The entire cryopump can be
enclosed in a housing 90 with a suitable flange 92 for
mounting to a vacuum chamber as is well known in the
art. A temperature sensor is often used to monitor the
refrigerator's second stage temperature.
Figure 1 shows the deposition characteristics for
argon on the various cryopanels. The deposition or
deposit being shown as 100, 101, 102, 103, and 104 on
panels 20, 30, 40, 50 and 70, respectively. Testing of
a cryopanel array according to figure 1 in a situation
intended to simulate its primary use in an argon sput-
tering application has shown the geometry of the figure 1
device to have an extremely high tolerance for large
quantities of cryo-deposited argon before any indica-
tions of a drop off in hydrogen pumping speed was
noted. At least 735,000 torr-liters of argon were
deposited before the hydrogen speed fell to 85% of its
initial value. It is believed that the excellent
performance of this cryopanel geometry is attributable
in large part to the conical silhouette wherein the
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base diameters of the individual cones increases linearly
from the cold end of the refrigerator toward the warm
end. The frontal surface area provided by the non-
overlapping outer region of each succeeding larger
conical section provides a site for the accumulation of
large quantities of argon. The deposition of argon in
these preferred zones delays the build up of argon in
the overlapping sections of adjacent tiers which are in
much closer proximi-ty to the charcoal surface normally
reserved for the adsorption of hydrogen. An illustration
of the probable argon deposition profile for the device
of U.S. Patent 4,295,338 is shown in figure 3 wherein
the argon deposit is identified as 110, 111, 112, 113,
and 114 on panels 115, 116, 117, 118 and 119, respectively.
In figure 1 and figure 3 argon deposition profiles it
will be noted that the deposition profile of the device
of figure 1 delays both the contamination of the charcoal
and the reduction of molecular conductance required to
insure sustained high hydrogen pumping speeds. The
prior art device of figure 3 shows contamination of
exposed adsorben-t or premature reduction of hydrogen
conductance by partial or total argon plugging. Vertical
alignment of the adjacent cryopanel surfaces, such as
shown in device of figure 3, create a condition in
which the bulk of cryo-deposited argon builds up at the
entrance of the flow passages leading to the hydrogen
adsorbent surfaces which are on the bottom of the
sloping sides of the panels 115, 116, 117, 118, and
119. This partial obstruction reduces the hydrogen
molecular conductance and thereby reduces the hydrogen
pumping speed significantly.
~ device according to the present invention used
with a cryogenic refrigerator cooling the cryopanel
array to 20K can be used in the Semiconductor Industry,
specifically for argon sputtering applications used in
the fabrication of large scale integrated circuits.
The primary gas species to be cryopumped are argon and
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hydrogen and occasionally air. The argon and air are frozen
out on the bare second-stage cryopanel sur-Eaces on the top o~
the individual cryopanels (20, 30, 40 50, 70) while the
hydrogen is adsorbed in charcoal granules which are epoxied to
the undersurfaces of the conical section of cryopanels ~0, 30,
~0, 5~ and 70. Cryopanels used in these applications must have
a high capacity for both aryon and hydrogen sothat regeneration
is required as infrequently as possible. The requirement for
high argon and hydrogen capacity, together with sustained high
hydrogen pumping speeds, typically requires a panel with both a
large bare surface area and a large charcoal-coated surface.
In addition, the charcoal sufaces must be fairly well protected
from contamination by cryo deposits oE argon or air in order to
maintain a high hydrogen capacity. The present invention
overcomes all of the prior art problems and provides the
required operating characteristics in order to be effective in
removing argon, air and hydrogen from the vacuum chamber.
Having thus described my invention~ what is desired
to be secured by letters patent is set forth in the appended
claims.
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